High voltage power cables using polyethylene or crosslinked polyethylene as insulator are provided with an internal semiconductive layer surrounding the outer periphery of the conductor and an external semiconductive layer surrounding the outer periphery of the insulator. Internal semiconductive layers relax heterogeneous electrical stress attributed to irregularities in the twisted wires in conductors, and also increase adhesion between the conductor and the insulator. External semiconductive layers homogenize electrical stress on the insulator surface.
Generally, for forming the semiconductive layers ethylene-based copolymer used as base polymer, such as ethylene-vinyl acetate copolymer and ethylene-ethyl acrylate copolymer, mixed with conductive carbon black are used.
When the conductive carbon black is not sufficiently dispersed in the base polymer, coarse grains are formed by coagulation of the carbon black particles. When the grains are formed in the interface between the insulator and the semiconductor layers, water tree deterioration initiates from these grains.
JP-A-59-8216 discloses a method of improving the water tree resistance by blending ethylene-propylene rubber with an ethylene-based copolymer, to prevent the formation of grains. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) According to this method, a large shearing stress is generated at the time of dispersing the conductive carbons in the polymer by using kneaders such as Bambury mixer so that the grains are finely divided to become harmless.
The above composition is, however, so viscous that a commonly used manufacturing method for high voltage cables, i.e., the common extruding method cannot be applied. In the common extruding method, the semiconductive layer and the insulator layer are simultaneously extruded on the outer surface of the conductor using a common crosshead. This method avoids disconformity between the semiconductor and the insulating layers. Since the common crosshead is used for the both layers, the extruding temperature of the semiconductive layer is restricted to be the same as that of the insulator layer or lower. When corsslinked polyethylene insulator is used, the temperature is limited to 130.degree. C. or less. In that case of the above composition comprising an ethylene-based copolymer blended with an ethylene-propylene rubber, the viscosity is extremely high so that extrusion becomes very difficult.